1. Field of the Invention
The present invention relates generally to an internal combustion engine and more specifically to an induction sensor arrangement which enables improved A/F control.
2. Description of the Prior Art
A previously proposed air-fuel ratio (A/F) control system for an internal combustion engine has been disclosed in an article entitled `Development of the Toyota Lean Combustion System` published in `NAINENN KIKAN` vol. 23 October 104 issue on pages 33 to 40. This system has been developed to enable control the A/F of the air-fuel mixture charged into the cylinders of the engine over a wide range spanning approximately stoichiometric to lean mixtures. To initially determine the amount of fuel which needs to be injected per cylinder, the output of an induction pressure sensor is used to sense how much air is being inducted into the engine. Subsequently, to complete the A/F control a specially developed air-fuel ratio sensor capable of sensing air-fuel ratios up until super lean mixtures are reached, is used.
In this system because the amount of fuel is supplied to the engine varies with the load thereon it is necessary to correct the output of the pressure sensor before using the same in the appropriate calculation or calculations. However, a problem is encountered in that, even though the effect of the pressure wave characteristics which occur in the induction system are anticipated and the pressure sensor constructed in a manner designed to compensate for the same, under given circumstances such as a sudden demand for acceleration the correlation between the sensor output and the actual air flow temporarily deteriorates.
As shown in FIG. 1, in the event that acceleration is required and the throttle valve opened quickly and the amount of air which is permitted to flow to the cylinders of the engine increased, the output of the pressure sensor does not increase for a period of 25-40 ms (by way of example) and thus does not accurately indicate the amount of air actually flowing through the system at that time. During this brief period as the amount of fuel being injected is determined in a microprocessor based on the output of the pressure sensor at or prior the beginning of the induction phase, the temporary discrepency between the actual amount of air entering the engine cylinders and that which is indicated by the pressure sensor results in the injection of insufficient fuel, the formation of an extremely lean mixture and a series of succesive misfires. This causes the engine to `stumble` increases the emission levels and deteriorates the driveability of the same undesirably.
In the event that the output of a flap type air flow sensor is used in place of the pressure sensor to sense the amount of air being inducted a similar problem is encountered. Viz., as shown in FIG. 1 for approximately 20 ms the output of the device remains unchanged and thereafter tends to undergo an increase which is far more rapid than the actual air flow increase (viz., overshoot). This tends to induce sudden leaning of the air-fuel mixture followed by an over enrichment thereof.